The invention relates to precision measuring apparatus, and particularly to apparatus for precisely measuring the location of points on working media. The invention is especially useful in the manufacture and/or testing of printed circuit boards (PCBs), and is therefore described below particularly with respect to this application, but it will be appreciated that the invention could advantageously be used in many other applications as well.
PCBs used in the electronics industry are continuously increasing in physical size, feature-density, and number of layers. For example, at the present time PCBs of up to 32".times.40", with up to 24 layers, are quite common. The high density of features (e.g., pads, conductors, holes, etc.) on each layer, the large number of layers, and the necessity for precise registration between the layers, require a very high accuracy in each individual layer. Since the production of each layer is based on a series of operations, such as plotting the artwork, developing the films, making contact prints on a laminate, developing, etching, etc., there will be an accumulation of errors in the various operations involved in the production of each layer. Therefore, in order to minimize rejections of a manufactured PCB, very strict quality control must be exercised during each stage of the production to ensure that each layer is maintained within very strict tolerances.
It is a rule of the art that the precision of the measuring apparatus must be at least one order of magnitude higher than the tolerance range required in any specific application. Under this rule, the PCB industry needs a measuring apparatus that can handle areas of a square meter or larger, and determine the coordinates of each point on it with a precision of a few microns.
One way used by the industry for obtaining relatively high precision is to adapt, for this application, the three-coordinate measuring machines which were originally designed for the metal industry. These machines depend for their precision on using a very sturdy, robust and heavy construction in order to maintain a high degree of three-dimension stability in space. Such machines, therefore, generally use granite beams, air bearings, and the like, and weigh more than four tons. Moreover, the machines used in the metal industry generally include mechanical-contact probes. Such probes are not suitable for measuring PCBs and therefore have to be replaced by non-contact probes or sensors, such as optical sensors. The resulting "hybrid" machine is not only extremely expensive and heavy, but is also not especially satisfactory for the PCB industry.